A number of control systems in which a master such as a PLC (=Programmable Logical Controller) or an IPC (=Industrial PC) controls a single or a plurality of controlled objects (slaves) via a network have been conventionally provided. For example, in a control system described in JP5394283, a ring topology is created by connecting a single master and a plurality of slaves (robots including a servo amplifier and a servo motor) using an EtherCAT (registered trademark) which is a real-time Ethernet. The master and the slaves include two communication ports, the ring topology is created by connecting these communication ports and a control packet generated by the master passes through all the slaves in order via the above communication ports and is turned over to return to the master again in an order opposite to the above one. This is repeated as one cycle and each slave reads an operation command included in the control packet and controls the robot.